Revisiting constraints on small scale perturbations from big-bang nucleosynthesis

We revisit the constraints on the small scale density perturbations (10⁴ Mpc^-1k10⁵ Mpc^-1) from the modification of the freeze-out value of the neutron-proton ratio at the big-bang nucleosynthesis era. Around the freeze-out temperature T~0.5 MeV, the universe can be divided into several local patches that have different temperatures since any perturbation that enters the horizon after the neutrino decoupling has not diffused yet. Taking account of this situation, we calculate the freeze-out value in detail. We find that the small scale perturbations decrease the n-p ratio in contrast to previous works. With the use of the latest observed ⁴He abundance, we obtain the constraint on the power spectrum of the curvature perturbations as ΔR²0.018 on 10⁴ Mpc^-1k10⁵ Mpc^-1.